Aromatic hydrocarbons, such as benzene, toluene and xylenes (collectively, “BTX”), serve as important building blocks for a variety of plastics, foams and fibers. Often these compounds are produced via catalytic reformation of naphtha through steam cracking of naphtha or gas oils, or other methods where substantial amounts of non-aromatic compounds are present.
The useful aromatic hydrocarbons may be separated from the non-aromatic hydrocarbons by, for example, solvent extraction. One widely used solvent extraction technique that is well known to those of ordinary skill in the art is the sulfolane process developed by UOP and Shell Oil Co. The process uses tetrahydrothiophene-1,1-dioxide (or sulfolane) as a solvent and water as a co-solvent to preferentially extract the desired BTX compounds from non-aromatic hydrocarbons. However, as is well known, other solvents having similar properties may also be used (e.g., glycols, N-FormylMorpholine (NFM) and N-Methyl-2-pyrrolidone (NMP)).
One embodiment of the sulfolane process uses extractive distillation, wherein a hydrocarbon feed containing aromatic and non-aromatic compounds is fed to a multistage distillation tower in which a circulating solvent (e.g., sulfolane) is present. The solvent changes the relative volatility of the aromatic/non-aromatic compounds such that aromatic compounds can be separated from the non-aromatic compounds. The non-aromatic compounds are fractionated overhead, while the aromatic compounds exit out the bottom of the tower along with the higher boiling solvent. The aromatic compounds are then separated from the solvent in another multistage column, such that the majority of the desired aromatic compounds are fractionated overhead while the majority of the solvent is recovered out the bottom of the tower. The solvent is then recycled back to the extractive distillation tower.
The presence of any heavy feed compounds (either aromatic or non-aromatic) entering the extractive distillation process through the fresh solvent, or any other stream that may be fed to the extractive distillation unit, poses a particular problem in that there is no escape avenue for these compounds in either the non-aromatic or aromatic fractions. As such, these heavy compounds, which distill in a similar range (co-boil) with the solvent, will accumulate in the solvent phase, which is detrimental to the performance of the process. An accumulation of heavy compounds in the solvent can cause an increase in the energy needed for separation of the aromatic and non-aromatic compounds, a decrease in unit capacity, a decrease in unit reliability, or a combination thereof. It is therefore desirable to remove these heavy compounds.
Traditionally, a regeneration (purification) process for an extractive distillation solvent is designed to remove very heavy byproduct compounds, such as thermal or oxidative degradation products of the solvent, as disclosed in U.S. Pat. No. 5,053,137. However, the present inventors have found that certain heavy feed compounds co-boil closely to the solvent under solvent regeneration conditions, and therefore, cannot be effectively removed using this process. Solutions proposed in the prior art include the use of adsorption beds, as disclosed in U.S. Pat. No. 8,552,247, or the use of a raffinate wash, as disclosed in U.S. Patent Application Publication No. 2010-0300939. Other techniques involve processing the extractive distillation solvent through a liquid-liquid extraction unit that is more effective at rejecting the heavy compounds, as disclosed by U.S. Pat. No. 8,860,358 and PCT Publication No. WO 2014/209585.
However, efficient and effective removal of the detrimental heavy compounds from the feedstocks prior to the extractive distillation process would eliminate the need for costly and energy intensive additional steps to purify the extractive distillation solvent.